The invention relates to a process for thermal treatment of residual materials containing oil and iron oxide in the form of oily sludges, e.g. sludges from steel mills.
Large quantities of sludges containing iron oxide, which are produced during cleaning of the cooling water from the continuous casting plants or mill trains, occur in the rolling mills in the steel industry. Depending on the granulometry of the iron constituents these sludges are contaminated by oil or grease residues. The oil or grease content increases the finer the granulometry of the constituents. The finest fraction ( less than 40 xcexcm), which may contain 14% oil, is particularly oily. Because of the high oil content it is difficult to return these sludges to the existing production line. An attempt was thus made to utilise these sludges in sintering plants. However, such high dioxin concentrations were measured during combustion of these sludges that this type of treatment is neither economical nor ecological. These sludges are therefore deposited in artificial lakes and thus pose a potential hazard to the environment, because oil and other residues may enter the ground water.
Typical compositions of these sludges are shown in the following table:
In this context xe2x80x9coilxe2x80x9d is generally understood to mean primarily lubricants and greases, which are used when rolling steel. Accordingly, they are chiefly hydrocarbons containing the various additives customary with these lubricants.
Document DE-C-552 387 relates to a process for the smelting of fine iron ores in a reduction furnace with six hearths. The multiple-hearth furnace is divided into two zones. In the first zone, formed by the top two hearths, the iron ore is initially pre-heated. To do this, hot gases of any suitable kind are supplied in an adjustable quantity. Burners may additionally be used, if appropriate. The second zone, comprising the bottom four hearths, is gastightly insulated from the pre-heating zone. No gas exchange therefore takes place between the two zones. The second zone is intended for the reduction of the iron ore. For this purpose reduction gases are supplied either in the bottom hearth or separately in each individual hearth. The top three hearths of the reduction zone are provided with muffles, to which heating gases are supplied. In this way the heating gases do not come into contact with the ores, and so the composition of the reduction gases can be altered independently of the temperature adjustment.
Consequently the task of the invention is to propose a process for thermal treatment of such residual materials containing oil and iron oxide.
According to the invention this problem is solved by a process for thermal treatment of residual materials containing oil and iron oxide in a multiple-hearth furnace with several hearths one above the other, in which the residual materials containing oil and iron oxide are mixed with a solid reducing agent, introduced continuously into the multiple-hearth furnace, placed on the top hearth and gradually transferred to the lower hearths, the residual materials containing oil and iron oxide being dried on the top hearths, and the oil subsequently evaporated and pyrolised. A gas containing oxygen is injected into the multiple-hearth furnace and reacts with part of the reducing agent to form reduction gas, the reduction gas reacting with the iron oxides to form directly reduced iron, the latter being discharged together with residues of reducing agents in the area of the bottom hearth in the multiple-hearth furnace.
An important advantage of the invention is that by-products are obtained from important constituents of the residual materials containing oil and iron oxide. The iron content can be returned after passing through the process into the production operations of the steel mill, the oil is pyrolised and the resulting pyrolysis gases are burnt. The oil thus contributes to generation of the necessary process heat. Ash consisting essentially of inert materials such as SiO2, Al2O3, MgO, etc. and possibly an excess of reducing agents may remain.
Sludge-type residual materials containing oil and iron oxide can be charged in this process, agglomeration of the particles being prevented by selective process control and continuous circulation. The process supplies a fine-grained end product regardless of the consistency of the feed material.
This is particularly advantageous if ash-forming reducing agents are used. As the solid end product is fine-grained, the ash can easily be separated from the iron. This separation can take place, for example, in the hot condition by screening.
After cooling below 700xc2x0 C. it is possible on the other hand to separate the reduced iron via magnetic separators from the ash and excess reducing agent. The quality of the directly reduced iron obtained in this way is virtually independent of the quantity of residues of the reducing agent.
The iron obtained can subsequently be processed into briquettes or introduced directly into a melting furnace (electric furnace, etc.) and further processed.
The reducing agent residues produced can be used with any unused reducing agents in a separate gasification reactor, the ash-forming constituents being advantageously separated as liquid slag and the crude gas formed used in the multiple-hearth furnace as combustion or reducing gas. Accordingly it is also possible to use a cheaper reducing agent with a relatively high ash content and/or work with a relatively high excess of reducing agent, which prevents agglomeration of the residual materials.
When working with excess reducing agents it is advantageous to process the residues in order to separate and reuse the unused reducing agents. This can be done, for example, by screening the residues, if the unused reducing agents are present in sufficiently coarse form. The unused reducing agents can be introduced directly into the multiple-hearth furnace.
However, part of the required reducing agent can also be deposited on one or more hearths at lower levels in the furnace.
It is thus possible that coarse-grained reducing agents (1-3 mm) are introduced at higher levels in the multiple-hearth furnace and fine-grained reducing agents ( less than 1 mm) further below. Consequently discharge of dust with the waste gases is largely avoided and the reaction accelerated by the fine reducing agent particles introduced further below.
Consumption of reducing agents is reduced by the charging of coarser particles, because the small particles are quickly consumed by reaction with H2O and CO2 from the waste gas in the upper hearths, on which an oxidising atmosphere prevails. The reduction gases in the furnace can be adjusted to an optimum concentration by selective feed of reducing agents in the lower hearths of the furnace with the result that a higher degree of metallisation can be achieved.
The process space is subdivided into different zones, the solids move continuously from the top downwards and the gases are conducted from the bottom upwards through the furnace. By subdividing the process space into different zones the process conditions in the different zones or even for each hearth can be measured and selectively influenced if required.
The residual materials containing oil and iron oxide are circulated continuously by rakes mounted on each furnace hearth and conveyed gradually to the underlying hearth.
Agglomeration of the reducing agents and residual materials containing oil and iron oxide is prevented by the continuous circulation. The rate of circulation depends on many factors such as the geometry of the rakes, the thickness of the layers, etc. The residual materials containing oil and iron oxide, the reducing agents and any reduced iron on the hearths should be circulated at least once every one to three minutes with the result that agglomeration is largely prevented.
Gases containing oxygen can be injected on the hearth, where the heat requirement must be covered by combustion of the excess process gases.
It is advantageous to use gases containing oxygen with a temperature of at least 250xc2x0 C.
A gaseous reducing agent can additionally be injected on the lowest hearths of the multiple-hearth furnace. This ensures more complete reduction of the oxides.
According to a further advantageous embodiment one or more hearths in the furnace are heated by burners.
In order not to reduce the concentration of reduction gases in the lower part of the furnace by flue gases of the heating system, energy can also be fed indirectly, i.e. by radiation heating, in this case.
According to another preferred embodiment gases are exhausted from the multiple-hearth furnace at one or more hearths. These hot gases can subsequently be passed through a CO2 scrubber to reduce the gas quantity and increase the reduction potential of the gas or through an additional reactor, in which carbon is present, so that the carbon dioxide present in the hot gases reacts with the carbon to form carbon monoxide according to Boudouard equilibrium and thus increases the reduction potential of the gas. The gases enriched with carbon monoxide are subsequently returned to the multiple-hearth furnace.
If necessary, additives are fed to one or more hearths in the lower section of the furnace.
In such a case it is advantageous to exhaust gases on a hearth above the hearth, on which additives are introduced.
According to a preferred embodiment gases are exhausted from the multiple-hearth furnace below a specific hearth and subsequently re-injected above this hearth into the furnace.
Iron oxide dusts or sludges containing carbon and metal can be introduced into the furnace at this hearth. As soon as they reach a certain temperature (about 900xc2x0 C.) the heavy metal oxides begin to react with the reducing agents whereby the heavy metals formed evaporate and are discharged together with the waste gases from the multiple-hearth furnace.
The heavy metals are advantageously exhausted on the hearths, where they are formed, and treated separately from the other waste gases.
The waste gases are subsequently oxidised, e.g. in an after-combustion chamber, the heavy metals being converted to heavy metal oxides, which can then be separated from the waste gases in filter equipment. Typical compositions of dusts and sludges containing heavy metal from electric or converter steel mills are shown in the following table.
The multiple-hearth furnace can be operated under a specific overpressure for a further increase in the productivity. In contrast to a rotary furnace, which is sealed via water seals with a diameter of about 50 m, this can be achieved very easily in a multiple-hearth furnace, which has only small seals on the drive shaft. In such a case pressure locks for the feed and removal of material must be provided.
According to another aspect of the present invention the use of a multiple-hearth furnace for thermal treatment of residual materials containing oil and iron oxide is proposed.
Further advantageous embodiments are listed in the sub-claims.